Waveguide channel diplexer and mode transducer

ABSTRACT

A waveguide channel diplexer in which plural resonant cavities are coupled to a segmented circular waveguide through thin slots on a plane longitudinal surface of the guide. The slots are located to preserve circular symmetry and consequently the purity of the circular electric mode; the cavities are designed to produce a complementary bandpass-bandstop filter pair. The diplexer structure may also be modified to operate as a mode transducer.

United States Patent Ren et a].

[ 51 June 6,1972

WAVEGUIDE CHANNEL DIPLEXER AND MODE TRANSDUCER Chung-Li Ren, Andover, Mass; Han-Chin Wang, Salem, NH.

Bell Telephone Laboratories, Incorporated, Murray Hill, NJ.

Apr. 16, 1971 Inventors:

Assignee:

Filed:

Appl. No.:

U.Si Cl. ..333/6, 333/21 R, 333/73 W,

333/83 R ..H01p 1/16, HOlp 5/12 ..333/6, 9, 21 R, 21 A, 73 W, 333/95 R, 98 R [56] References Cited UNITED STATES PATENTS 3,112,460 11/1963 Miller ..333/9 3,321,720 5/1967 Shimada ..333/21 R X Primary Examiner--Paul L. Gensler AttorneyR. J. Guenther and E. W. Adams, Jr.

[5 7] ABSTRACT A waveguide channel diplexer in which plural resonant cavities are coupled to a segmented circular waveguide through thin slots on a plane longitudinal surface of the guide. The slots are located to preserve circular symmetry and consequently the purity of the circular electric mode; the cavities are designed to produce a complementary bandpass-bandstop filter pair. The diplexer structure may also be modified to operate as a mode transducer.

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. C. REN /Nl/E/VTO;$ WANG V @Jmq ATTORNEY PATENTEDJUH 6 m2 SHEET 2 OF 3 FIG. 3

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PATENTEDJUH s 1972 WAVEGUIDE CHANNEL DIPLEXER AND MODE TRANSDUCER BACKGROUND OF THE INVENTION This invention relates to electromagnetic waveguide transmission and more particularly to waveguide mode transducing devices, especially a multipole complementary bandpassbandstop filter which may be used for channel dropping or channel combining in a millimeter wave transmission system.

The TE circular electric mode of wave propagation in circular waveguides has received considerable attention due to its recognized low-loss propagation characteristic which exhibits decreasing loss with increasing frequency. The problem of multiplexing in a communication system using this mode of wave propagation was first considered by E. A. .I. Marcatili in his article entitled Mode-Conversion Filters," published in the January 1961 issue of the Bell System Technical Journal, pages l49-l 84. The filters described by Marcatili in this article, and in his U.S. Pat. No. 2,963,663, use high loss-narrow bandwidth coaxial cavities.

The high loss of a coaxial cavity, which is formed from a rectangular waveguide wrapped around a cylinder such as a circular waveguide wall, causes the overall filter loss to be considerable if many such cavities are used. Some of the filters utilize circular electric mode conversion resonators to achieve channel separation. These exhibit an extremely low loss, but provide only a single bandpass section, and for multipole arrangements, additional bandpass sections of the coaxial cavity type are required. This, of course, introduces substantial loss and severe bandwidth limitations.

Coupling to the coaxial cavity is provided through a distributed arrangement of apertures in the circumferential wall. Due to the circular geometry, it is difficult and costly to fabricate the cavities and the appropriate aperture arrangement. In addition, both the mode conversion resonator and the coaxial resonator exhibit relatively narrow frequency bands which are free from spurious resonance. In certain transmission systems, it is essential to have channel diplexers capable of operating over as broad a bandwidth as possible and many system applications require a band broader than these cavities provide. lmprovementswhich would increase bandwidth and simplify the structural arrangement would make the device more attractive.

The desirable TE mode, which exhibits a longitudinal component that is exclusively magnetic (H and is therefore alternatively designated I-P may be propagated in a segmented circular waveguide. As used herein, a segmented circular waveguide means a waveguide having a cross-section which is a segment of a circle, such as a semicircle, and the resulting chordal surface is the plane surface containing the chords of the cross-sectional segment, or the diameters if the segment is a semicircle. The longitudinal magnetic component H is maximum at the center line of the chordal surface, and it is thus possible to couple directly to this mode at this central location. There have been previous attempts to couple along a plane surface of a segmented waveguide such as is disclosed in U.S. Pat. No. 3,112,460 to S. E. Miller. However, these arrangements require bifurcating a circular guide into two parallel semicircular guides and coupling equal amounts of incident energy into each. Both of the parallel semicircular guides are coupled to the output port with critical dimensions arranged so that the half of the energy arriving at the output guide from one of the semicircular guides is out-of-phase with the other half of the energy arriving from the other semicircular guide, thereby preventing any of the incident energy from coupling to the output guide. A frequency selective reflecting device in each of the semicircular guides causes energy at the selected frequency to return to the apertures and a 180 phase delay must be provided in one guide to insure the desired coupling to the output port. This complex arrangement is, of course, difficult to fabricate.

SUMMARY OF THE INVENTION In accordance with the present invention, a channel diplexer is provided which offers fabrication simplification and broader bandwidth than the previous filter designs without significantly sacrificing the low-loss characteristics of the circular electric mode propagation. The main through waveguide is a single segmented waveguide and cavities are mounted directly on the chordal surface. The cavities which are preferably complementary bandpass and band rejection filters, are tuned to resonate at a specified channel frequency. Coupling to the longitudinal magnetic component is made along the center line of the chordal surface by means of a single aperture for each cavity. Since all the coupling apertures are located on a plane surface, rather than in the more complex circumferential distribution required to couple to coaxial cavities, a substantial cost reduction over that type of filter is realized; there is also a substantial reduction over the cost of the parallel semicircular arrangement.

A segmented circular guide exhibits a low through loss characteristic similar to that of a circular guide. Through loss in a circular waveguide is slightly lower than it is in a segmented guide of the same diameter, but the loss in the segmented guide is still effectively negligible and is a small price to pay for the large increase in bandwidth offered by the segmented structure. In one typical system application, the input wave in the main waveguide could have a percentage band width greater than 15 percent. These bandwidths are substantially higher than can be provided if the main waveguide were circular in cross-section. v

The channel diplexer is envisioned as part of a tandem arrangement of numerous similar diplexers in a millimeter wave multiplexer. It may, of course, have other applications and the principles of the invention can also be used to design a mode transducer.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of a channel diplexer in accordance with the present invention.

FIG. 2 is a perspective view of an alternative embodiment of the channel diplexer in accordance with the present invention.

FIG. 3 is a perspective view of another version of the channel diplexer of FIGS. 1 and 2.

FIG. 4, included for purposes of explanation, is a diagram of a multiplexer, incorporating diplexers, such as are illustrated in FIGS. 1, 2 or 3, as both channel dropping filters and channel combiners.

FIG. 5 is a perspective view of a mode transducer in accordance with the present invention.

DETAILED DESCRIPTION FIG. 1 illustrates a two-pole channel diplexer in accordance with the invention comprising a two-cavity branching filter 20 and a two-cavity band rejection filter 30 longitudinally disposed along segmented waveguide 10. Waveguide 10 is proportioned to support only the lowest order circular electric mode, i.e., the I-I mode, over a range of frequencies, including channel bands centered at f,, f j;,. Hereinafter, this range is referred to as the subband, and each channel is designated by its center frequency.

Segmented circular waveguide 10 is conductively bounded by circumferential wall 11 and plane chordal surface 12. In order to preserve the circular symmetry of the electric and magnetic field patterns propagating in waveguide 10 and hence avoid unwanted moding, the waveguide is designed so that chordal surface 12 contains the diameter of the circular cross-section; i.e., waveguide 10 is a semicircular waveguide.

In a millimeter wave multiplexer, such as is shown in FIG. 4, a large number of channel diplexers are arranged in tandem. In such a system it is preferable to have as wide a subband bandwidth as possible and if the diameter of waveguide 10 is properly chosen a percentage bandwidth (free of cut-ofi for any mode) which is approximately 18 percent may be established. The subband is located between the cut-ofi" frequencies of the higher order circular modes H and It is noted that the circular magnetic mode 15 would prevent such a wide bandwidth; this mode is present in circular guides, but is not supported by the segmented guide. Therefore, the subband propagating in waveguide 10 is wider than that in a circular guide.

The H mode propagates in the segmented waveguide with an extremely low-loss characteristic. The magnitude of the loss is slightly greater in a segmented guide than in a circular guide of the same diameter, due to the additional loss from the induced surface currents on the flat chordal surface. The increased loss is, however, extremely small and even if it were on the order of twice as large, the channel diplexer loss would still be insignificant relative to the loss resulting from the use of the more conventional rectangular waveguide.

The diplexer of FIG. I is shown and described as a channel dropping filter with the subband signal f}, j; f entering Input Port 1, the dropped channel of frequency f} propagating out at Output Port 2, and the remainder of the subband signal after the dropped channel has been removed propagating out at Through Port 3, but the diplexer is, of course, reciprocal and may also be used as a channel combiner.

The incoming subband signal containing a combination of channels propagates in the H" mode. The channel which is to be dropped is coupled into branching filter 20. Branching filter 20, together with the two-cavity band rejection filter 30, forms a two-pole filter of complementary design. Thus, the through channels f f f,, fl f,, will propagate down waveguide 10 under matched conditions.

Branching filter contains a single tube 23 divided by window iris 21 into two cavities 24 and 25, and rejection filter 30 contains two similar cavities 3 l and 32. Cavities 25, 31 and 32 are end-coupled to waveguide 10 through coupling apertures l6, l7 and 18, respectively, located on chordal surface 12. These coupling apertures are oriented along the center longitudinal axis 13 of surface 12 so that the presence of these apertures preserves the circular symmetry of the structure with respect to the fields of the incoming waves of H mode. In order to avoid moding problems, the apertures are slots which are thin relative to the diameter of waveguide 10. These slots, which may be rectangular or elliptical, couple only the circular H mode to the cavities. Since H is below cut-off for m a 2, only H will be excited and the diplexer will not respond to incoming signals in any spurious modes.

The longitudinal magnetic field in waveguide 10 is maximum along center line 13 of surface 12, and since thin slot apertures l6, l7 and 18 are located along this line, strong coupling is provided. The magnetic field at this location is, for example, several times larger than the longitudinal magnetic field of the Hm" mode in the circular waveguide on its circumferential waveguide wall.

Branching filter 20 and band rejection filter 30 are dimensioned so that their cavities resonate at f}, the center frequency of the channel to be dropped. This is accomplished by making the electrical length of the cavity approximately an integral multiple of one-half M; A, is the cavity guide wavelength at f,. Thus, the desired channel f, is coupled to Output Port 2 and the rejection cavities turn back energy in this frequency band, preventing its passage to Through Port 3. The bandpass and band rejection filters are designed as a complementary pair and each has the conventional maximally flat frequency response characteristic within be operating subband, so that constant resistance will be seen looking into input Port 1.

Due to the advantages provided by the direct coupling to the H mode in waveguide 10 by means of the thin slots on surface 12, the resonance cavities may have any physical size and cross-section, such as circular (shown in FIG. 1), rectangular (shown in FIG. 2), elliptical, coaxial or semicircular. However, whatever shape and dimensions are chosen the cavity must have one of its magnetic field components aligned with the thin coupling slot in the common wall between the cavity and chordal surface 12. In practice, the circular and rectangular cavities, which have well established properties and design data, are preferred, and the appropriate choice of resonant modes in the cavities of both filter sections 20 and 30, will be the H mode for circular cross-sections and H for the rectangular cross-sections. In generaL'the resonating H mode is designated H, and the resonating H mode is designated H where 12/2 A corresponds to the approximate length ofthe cavities.

Any oversized (overmoded) waveguide may be used for the resonance cavities to achieve a higher intrinsic Q for the resonant mode which may be either the dominant or higher order mode. However, if the cavity diameter is large enough to allow higher order modes, frequencies in the subband f f outside the desired channel band f, may be resonant for such higher modes. Therefore, for broad subband bandwidth, the cavity dimensions should be chosen so that no spurious resonance exists at frequencies within the subband. The dominant mode H or H will provide the widest operating bandwidth.

Tube 23 is arranged as a bandpass section. In the two-pole design of FIG. 1 it includes two resonant cavities 24 and 25 to complement the two rejection cavities 31 and 32. Where n l, the electrical length of each cavity 24 and 25 is /2. The cavities are mutually end-coupled by window iris 21 which comprises a conductive surface with a small central coupling hole 22. The type of coupling will depend upon design requirements and alternatively, other obstacles such as a single conductive post extending transversely across the cavity perpendicular to slot 16, may be used to couple cavities 24 and 25. As is well known by one skilled in the art, the discontinuities of the obstacle will alter the electrical length of the cavities and thus the physical lengths h and k, of cavities 25 and 24, respectively, may be slightly different from )tJZ and from each other.

The energy at frequency f, in the resonant mode may be coupled to Output Port 2 in any physical manner so long as it is compatible with the electrical design and causes no moding problems. The simplest form is a straight connection through aperture 27. Output Port 2 may be either a rectangular waveguide 26, which supports the H F' mode, or a waveguide with any other desired cross-section.

The dimensions of the pair of band rejection cavities 31 and 32 are chosen to support the same mode and resonant frequency as is supported by bandpass cavities 24 and 25. The three cavity tubes are separated by a center-to-center distance L which is an odd integral number times N 14; A" is the guide wavelength of the Hm mode in segmented waveguide 10. Rejection cavities 31 and 32 are each terminated by a short positioned so that energy at the desired frequency is turned back prior to reaching Through Port 3. The termination may be simply a conductive plate 33 and 34 spaced from surface 12 by a distance In,- which is approximately the length n/2 A In addition, adjustable tuning elements, such as inductive tuning screws, may be inserted longitudinally through plates 33 and 34. Alternatively, capacitive tuning screws could be inserted transversely through the cavity walls perpendicular to slots 17 and 18. Band rejection cavities 31 and 32 may use either type of tuning element but bandpass cavities 24 and 25 can, because of their structural arrangement, be tuned only by the latter mechanism.

A diplexer designed in accordance with the present invention is capable of realizing any multiple configuration. For example, a three-pole design of the diplexer configuration in accordance with this invention, requires only the addition of one bandpass cavity following cavity 24 and one band rejection cavity downguide from cavity 32, provided that the three bandpasscavities and the three band rejection cavities are designed as a complementary pair. The multipole modification offers no apparent manufacturing difficulties and the diplexer is in this respect superior to the circular waveguidecoaxial cavity type of filter.

FIG. 2 is an alternative form of the diplexer of FIG. 1. The structures of the two differ only in the shape of the resonant cavities and like numerals indicate like elements. The narrow dimension w and broad dimension 1 of rectangular prisms 43, 44 and 45 are a matter of design choice, but is preferable that they be chosen to optimize the loss and bandwidth requirements of the system. Prisms 44 and 45 serve as band rejection cavities, and prism 43 is separated into two bandpass cavities 46 and 47 by post 48 which is a conductive post conductively secured to the central points of the broad walls of that prism. The operation of the configuration in FIG. 2 is substantially identical to that of FIG. 1, except that the H P mode is used.

The high profile of the multiple diplexer of FIGS. 1 and 2 may be reduced by placing the narrow sidewalls (the walls wounded by the dimensions w and h of the cavities along surface 12. The end coupling would thus be replaced by longitudinal coupling. Such an arrangement is the subject of a ropending application of G. A. Tuchen, Ser. No. 134,806, filed on an even date herewith and assigned to the assignee hereof, and is shown in FIG. 3.

In FIG. 3 the cavities are rectangular in cross-section and this diplexer is similar to the structure in FIG. 2 so that like numbers indicate like elements. The singular difference between the two embodiments is that the cavities are positioned longitudinally along surface 12 and thus in FIG. 3 the dimension which determines the resonant frequency, the length h of the cavity, is oriented parallel to the propagation in waveguide 10.

The filter cavities may, of course, be dimensioned to support either the dominant or higher order modes. The most downguide rejection cavity 52 is shown having a length I1 twice that of h,., the length of rejection cavity 51. Cavity 52 supports theH -F mode, while cavity 51 supports the H F mode. Likewise, bandpass filter 20 may be dimensioned for higher order modes and cavity 50 has a length 11,, substantially equal to k The second bandpass cavity 53 also supports the Hmz mode and is positioned upguide from and inductively coupled through their mutual end wall 54 to cavity 50. Coupling slots l6, l7 and 18 are located as in FIG. 2; that is, they are separated by L which is an odd multiple of onequarter A, The increased length of the higher order cavities does not alter the relative location of the slot and the end wall of the cavity. Cavity 53 is coupled to waveguide only through cavity 50. It is anticipated that output waveguide 26, which is coupled inductively to cavity 53, will continue the narrow dimension w of the cavities, and guide 26 may thus be oversized in width and require a transducer for conversion to standard waveguide.

FIG. 4 illustrates a multiplexer in which channel diplexers 60 and 70 in accordance with the invention, are arranged in tandem. A broadband input of 75 to 110 Gl-Iz at Port A is divided by a series of band diplexers 71 into subbands. The subbands are then applied to a channel bank, such as 64, in which a serial arrangement of channel dropping filters 70 each drops a selected frequency band of approximately 500 MHz to a terminal device 65, which may be a channel terminal or a repeater. The channel signal derived from device 65 (either an amplification of the input if it is a repeater or a generated signal if it is a channel terminal) is then combined by an identical arrangement of channel combiners 60 and band diplexers 61 to form the output of Port B. The diplexers of the present invention are extremely well suited to the broadband multiplexer because of their individual broadband characteristics.

The multiplexer also includes low-pass filters 66 where appropriate. It is noted that if the connective waveguides in the multiplexer are circular, a simple circular to semicircular taper can be built for the transition from circular waveguide to diplexers 60 and 70. The multiplexer operates reciprocally for another frequency band as indicated by the 4075 GI-Iz broadband input at Input B. Y

The channel diplexers as shown in FIGS. 1, 2 and 3 may also be used as mode transducers between the mode of the signal at Output Port 2 and the l-I mode at Input Port 1, when Through Port 3 is terminated by a matched load. (This port may be left open since the H mode offers a good match to air.) Alternatively, the mode transducer can also be designed as illustrated in FIG. 5. The two band rejection cavities and the matched load at Through Port 3 is replaced with a variable short which can be used to provide tuning. It is essentially the bandpass section of FIG. 1 and like elements are indicated by like numbers.

In all cases it is to be understood that the above-described arrangements are merely illustrative of a small number of the many possible applications of the principles of the invention. Numerous and varied other arrangements in accordance with these principles may readily be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

l. A waveguide transmission device comprising a conductively bounded waveguide having a curved surface and a single plane surface and capable of propagating energy in a circular electric mode, a bandpass cavity capable of supporting a resonant mode and mounted on the single plane surface of the waveguide, and means comprising a single aperture in the single plane surface for exclusively coupling energy between the circular electric mode in the waveguide and the resonant mode in the bandpass cavity.

2. A waveguide transmission device comprising a conductively bounded segmented circular waveguide having a circumferential surface and a plane chordal surface parallel to the axis of the waveguide and capable of propagating energy in a circular electric mode, a primary bandpass cavity capable of supporting a resonant mode, a pair of band rejection cavities each capable of supporting a resonant mode, said primary bandpass and each of said band rejection cavities being mounted on the plane chordal surface, apertures in the plane chordal surface for coupling energy between the circular electric mode in the waveguide and the resonant modes in said primary and each of said band rejection cavities, a secondary bandpass cavity capable of supporting a resonant mode and coupled to said primary bandpass cavity, said secondary bandpass cavity being designed so that the bandpass and band rejection cavities function as a complementary pair, and an output port coupled to said secondary bandpass cavity.

3. A transmission device as claimed in claim 2 wherein said apertures are thin rectangular slots lying on the center line of the chordal surface.

4. A transmission device as claimed in claim 2 wherein said chordal surface contains a cross-sectional diameter of the waveguide.

5. A transmission device as claimed in claim 2 wherein said primary bandpass cavity has a circular cylindrical shape, is dimensioned to support the ri mode, and is mounted with its axis perpendicular to the chordal surface.

6. A transmission device as claimed in claim 2 wherein the primary cavity is a rectangular prism and is dimensioned to support the H P mode.

7. A transmission device as claimed in claim 2 wherein each of said bandpass and band rejection cavities is dimensioned to resonate in a selected frequency band.

8. A transmission device as claimed in claim 7 wherein the electrical lengths of each of said bandpass and band rejection cavities are each integral multiples of one-half the cavity guide wavelength of the center frequency of the selected frequency band.

9. A transmission device as claimed in claim 7 wherein said primary bandpass cavity is end-coupled to said secondary bandpass cavity and said primary cavity and each of said band rejection cavities are mounted on the chordal surface and endcoupled to the waveguide by a thin slot parallel to the axis of the waveguide and lying on the center line of the chordal surface.

10. A transmission device as claimed in claim 7 wherein each of said bandpass and band rejection cavities has a circular cylindrical shape, is dimensioned to support the I-I mode, and is mounted with its axis perpendicular to the chordal surface.

11. A transmission device as claimed in claim 7 wherein each of said bandpass and band rejection cavities is a rectangular prism and is dimensioned to support the H mode.

12. A transmission device comprising a semi-circular waveguide capable of supporting the H circular electric mode, a bandpass circular cylindrical cavity mounted on the plane chordal surface of the waveguide and capable of supporting a dominant resonant mode in a selected frequency band, a pair of band rejection circular cylindrical cavities mounted on the chordal surface and each capable of supporting the dominant resonant mode in the selected frequency band, and at least one aperture for each cavity located on the chordal surface for coupling between the H circular electric node of the waveguide and the dominant resonant mode of the cavities.

13. A transmission device comprising, a semi-circular waveguide capable of supporting the H circular electric mode, a bandpass cavity of rectangular cross-section mounted on the plane chordal surface of the waveguide and capable of supporting a dominant resonant mode in a selected frequency band, a pair of band rejection cavities of rectangular crosssection mounted on the chordal surface and each capable of supporting the resonant mode in the selected frequency band, and at least one aperture for each cavity located on the chordal surface for coupling between the H circular electric mode of the waveguide and the dominant resonant mode of the cavities.

14. A transmission device comprising, a segmented circular waveguide capable of supporting the H circular electric mode, said waveguide having a plane chordal surface parallel to the axis of the waveguide, a bandpass cavity mounted on the chordal surface and capable of supporting a resonant mode, an adjustable temlinating short positioned in one end of the waveguide, and one aperture located on the chordal surface for coupling between the H circular electric mode of the waveguide and the resonant mode of the cavity, and an output port coupled to the bandpass cavity so that energy in the H circular electric mode in the waveguide is transferred to the output port in another mode. 

1. A waveguide transmission device comprising a conductively bounded waveguide having a curved surface and a single plane surface and capable of propagating energy in a circular electric mode, a bandpass cavity capable of supporting a resonant mode and mounted on the single plane surface of the waveguide, and means comprising a single aperture in the single plane surface for exclusively coupling energy between the circular electric mode in the waveguide and the resonant mode in the bandpass cavity.
 2. A waveguide transmission device comprising a conductively bounded segmented circular waveguide having a circumferential surface and a plane chordal surface parallel to the axis of the waveguide and capable of propagating energy in a circular electric mode, a primary bandpass cavity capable of supporting a resonant mode, a pair of band rejection cavities each capable of supporting a resonant mode, said primary bandpass and each of said band rejection cavities being mounted on the plane chordal surface, apertures in the plane chordal surface for coupling energy between the circular electric mode in the waveguide and the resonant modes in said primary and each of said band rejection cavities, a secondary bandpass cavity capable of supporting a resonant mode and coupled to said primary bandpass cavity, said secondary bandpass cavity being designed so that the bandpass and band rejection cavities function as a complementary pair, and an output port coupled to said secondary bandpass cavity.
 3. A transmission device as claimed in claim 2 wherein said apertures are thin rectangular slots lying on the center line of the chordal surface.
 4. A transmission device as claimed in claim 2 wherein said chordal surface contains a cross-sectional diameter of the waveguide.
 5. A transmission device as claimed in claim 2 wherein said primary bandpass cavity has a circular cylindrical shape, is dimensioned to support the H*11n mode, and is mounted with its axis perpendicular to the chordal surface.
 6. A transmission device as claimed in claim 2 wherein the primary cavity is a rectangular prism and is dimensioned to support the H10n mode.
 7. A transmission device as claimed in claim 2 wherein each of said bandpass and band rejection cavities is dimensioned to resonate in a selected frequency band.
 8. A transmission device as claimed in claim 7 wherein the electrical lengths of each of said bandpass and band rejection cavities are each integral multiples of one-half the cavity guide wavelength of the center frequency of the selected frequency band.
 9. A transmission device as claimed in claim 7 wherein said primary bandpass cavity is end-coupled to said secondary bandpass cavity and said primary cavity and each of said band rejection cavities are mounted on the chordal surface and end-coupled to the waveguide by a thin slot parallel to the axis of the waveguide and lying on the center line of the chordal surface.
 10. A transmission device as claimed in claim 7 wherein each of said bandpass and band rejection cavities has a circular cylindrical shape, is dimensioned to support the H*11n mode, and is mounted with its axis perpendicular to the chordal surface.
 11. A transmission device as claimed in claim 7 wherein each of said bandpass and band rejection cavities is a rectangular prism and is dimensioned to support the H*10n mode.
 12. A transmission device comprising a semi-circular waveguide capable of supporting the H01 circular electric mode, a bandpass circular cylindrical cavity mounted on the plane chordal surface of the waveguide and capable of supporting a dominant resonant mode in a selected frequency band, a pair of band rejection circular cylindrical cavities mounted on the chordal surface and each capable of supporting the dominant resonant mode in the selected frequency band, and at least one aperture for each cavity located on the chordal surface for coupling between the H01 circular electric mode of the waveguide and the dominant resonant mode of the cavities.
 13. A transmission device comprising, a semi-circular waveguide capable of supporting the H01 circular electric mode, a bandpass cavity of rectangular cross-section mounted on the plane chordal surface of the waveguide and capable of supporting a dominant resonant mode in a selected frequency band, a pair of band rejection cavities of rectangular cross-section mounted on the chordal Surface and each capable of supporting the resonant mode in the selected frequency band, and at least one aperture for each cavity located on the chordal surface for coupling between the H01 circular electric mode of the waveguide and the dominant resonant mode of the cavities.
 14. A transmission device comprising, a segmented circular waveguide capable of supporting the H01 circular electric mode, said waveguide having a plane chordal surface parallel to the axis of the waveguide, a bandpass cavity mounted on the chordal surface and capable of supporting a resonant mode, an adjustable terminating short positioned in one end of the waveguide, and one aperture located on the chordal surface for coupling between the H01 circular electric mode of the waveguide and the resonant mode of the cavity, and an output port coupled to the bandpass cavity so that energy in the H01 circular electric mode in the waveguide is transferred to the output port in another mode. 